How to implement detailed internal error logging

[pemattern]

I have implemented IntoResponse for my AppError type. This is great for generating adequately generic error responses without exposing sensitive information. However, I want to interally log a detailed error message using tracing::error!(). Currently, I'm using a clumsy approach of just having a side-effect in my conversion from AppError to Response. Is there a better/cleaner way to do this, with out having to add .map_err(... tracing::error!(...)) into all my handlers?

Thank you!

Edit:
I have noticed that the error handling example handles internal error logging in exactly the same way. Is this simply the idiomatic axum-way? Is there potentially a need for a .with_side_effect(...) api for a cleaner separation of concerns (i.e. no side effects in an infallible conversion)?

[mladedav]

Doing it as side-effects in the IntoResponse implementation is currently the easiest solution.

Another option people sometimes do is adding additional information (like the error itself) into response extensions and then checking the extensions in a middleware.

Personally, I would advise against the map_err in handlers way because remembering to do that at every single place including all short-circuiting ? would be almost impossible for any mid-sized project. Or maybe I'm just a pessimist.

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What would you say is the ideal scenario? How would you want it to work if anything was technically possible? This is an area that I think could be improved and it would be great to have opinions from users.

[pemattern]

Thank you for your reply. I have added the AppError into the Response's Extensions and can now log them from a dedicated middleware, great!

This has ultimately achieved my desired goal of: 1) logging errors without repeating myself in every handler and 2) separating concerns of the type conversion and logging.

One minor issue that I faced was that an Extension must derive Clone which isn't the case for some of the Error types of crates I'm using, sqlx::Error being the culprit in my case. So just wrapped it in an Arc, in my AppError and that's fine, I guess.

Using Extensions + middleware::from_fn is a great method for applying side effects to handlers. I don't think any change is required, I simply wasn't aware of this pattern. Maybe I could modify the error handling example to use this pattern?

[mladedav]

Sounds good, a PR would be welcome.

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For context, before 1.0 the clone bound wasn't there, but then it's impossible to clone the request/response even if you can handle the body type and there's no way around that that I know of. So for example retry layers trying to clone requests would lose all extensions.